This invention relates to a lithium ion conductive glass-ceramics suitable for use as wholly solid electric cells, gas sensors and electrochemical devices of various types, and electric cells and gas sensors using such glass-ceramics.
Recent development in electronics has brought about high-performance electronic devices of a compact and light-weight design and, as a power source of such electronic devices, development of an electric cell of a high energy density and a long life is strongly desired for.
Lithium has the highest oxidation-reduction potential of Li/Li+ of all metal elements and has the smallest mass per 1 mol and, therefore, a lithium cell can provide a higher energy density than other types of cells. Moreover, if a lithium ion conductive solid electrolyte is used, this electrolyte can be made very thin and, therefore, a cell of a thin film can be formed and increase in energy density per unit volume can thereby be realized.
A lithium ion cell which has been realized to date uses an organic electrolyte solution as its electrolyte and this makes it difficult to achieve a cell of a compact design such as a thin film design. This lithium ion cell has additional disadvantages that it has likelihood of leakage of electrolyte solution and likelihood of spontaneous combustion. If this lithium ion cell is replaced by a cell employing an inorganic solid electrolyte, a wholly solid cell of a high reliability will be realized.
Carbon dioxide gas produced by combustion of fossil fuel is a main cause of a hothouse effect which has recently become a serious problem and it has become necessary to incessantly watch the concentration of carbon dioxide gas. Therefore, establishment of a system for detecting carbon dioxide gas is a matter of increasing importance for the maintenance of a comfortable life in the future human society.
Carbon dioxide gas detection systems which are currently in use are generally of a type utilizing absorption of infrared ray. These systems, however, are large and costly and besides are susceptible to contamination. For these reasons, studies have recently been actively made to develop a compact carbon dioxide gas sensor using a solid electrolyte. Particularly, many reports have been made about studies using a lithium ion solid electrolyte.
For realizing such gas sensor using solid electrolyte, development of a solid electrolyte which is highly conductive, chemically stable and sufficiently heat proof is indispensable.
Among known electrolytes, Li3N single crystal (Applied Physics Letters, 30(1977) P621-22), LiIxe2x80x94Li2Sxe2x80x94P2S5 (Solid State Ionics, 5(1981) P663), LiIxe2x80x94Li2Sxe2x80x94SiS4 (J. Solid State Chem. 69(1987) P252) and LiIxe2x80x94Li2Sxe2x80x94B2S3 (Mat. Res. Bull., 18(1983) 189) glasses have high conductivity of 10xe2x88x923S/cm or over at room temperature. These materials, however, have the disadvantage that preparation of these materials is difficult and these materials are not chemically stable and not sufficiently heat proof. Particularly, these materials have the fatal disadvantage that decomposition voltage of these materials is so low that, when they are used for an electrolyte of a solid cell, a sufficiently high terminal voltage cannot be obtained.
An oxide lithium solid electrolyte does not have the above described disadvantages and has a decomposition voltage which is higher than 3V and, therefore, it has possibility of usage as a wholly solid lithium cell if it exhibits a high conductivity at room temperature. It is known in the art that conductivity in an oxide glass can be increased by increasing lithium ion concentration. However, there is limitation in increasing the lithium ion concentration even if rapid quenching is employed for glass formation and conductivity of this glass at room temperature is below 10xe2x88x926S/cm at the highest.
Japanese Patent Application Laid-open Publication No. Hei 8-239218 discloses a gas sensor using a thin film of a lithium ion conductive glass. The conductivity of this lithium ion conductive glass thin film is within a range from 1.7xc3x9710xe2x88x927 S/cm to 6.1xc3x9710xe2x88x927 S/cm. This is not a sufficiently high value and a solid electrolyte having a higher conductivity is desired for.
There are many reports about oxide ceramics (sintered products) having a high conductivity. For example, Li4GeO4xe2x80x94Li3VO4 exhibits conductivity of 4xc3x9710xe2x88x925 S/cm at room temperature (Mat. Res. Bull. 15(1980) P1661), and Li1+XAlXGe2xe2x88x92X(PO4)3 exhibits conductivity of 1.3xc3x9710xe2x88x924 S/cm at room temperature (Proceedings of 8th International Meeting on Lithium Batteries, Jun. 6-21, 1996, Nagoya, Japan P316-317). Oxide ceramics are superior in conductivity to oxide glasses but have the disadvantages that they require a complicated and troublesome process for manufacturing and that they are difficult to form, particularly to a thin film.
In short, the prior art lithium ion solid electrolytes have the problems that they are either low in conductivity, hard to handle or hard to form to a compact design such as a thin film.
It is, therefore, an object of the invention to provide glass-ceramics which have solved these problems and exhibit a high lithium ion conductivity at room temperature.
It is another object of the invention to provide a lithium electric cell and a gas sensor of a high performance by utilizing such glass-ceramics.
As described above, ceramics exhibit conductivity of 10xe2x88x924S/cm or over at room temperature. These ceramics, however, have pores and a large grain boundary which cannot be eliminated completely and existence of these pores and grain boundary results in decrease in conductivity. If, therefore, glass-ceramics including the above crystal are provided, there will be no pores and the grain boundary will be improved and, as a result, a solid electrolyte having a higher conductivity is expected to be produced. Besides, glass-ceramics which share a feature of glass can be easily formed into various shapes including a thin film by utilizing this feature of glass. For these reasons, glass-ceramics are considered to have practical advantages over ceramics made by sintering.
As a result of studies and experiments made by the inventor of the present invention on the basis of the above described basic concept, the inventor has succeeded in obtaining glass-ceramics having a high lithium ion conductivity at room temperature by producing glasses including ingredients of P2O5, SiO2, GeO2, TiO2, ZrO2, M2O3 (where M is en element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Al2O3, Ga2O3 and Li2O and causing a crystal phase of conductive crystal Li1+X(M, Al, Ga)X(Ge1xe2x88x92XTiY)2xe2x88x92X(PO4)3 (where 0 less than Xxe2x89xa60.8 and 0xe2x89xa6Yxe2x89xa61.0) to precipitate from the glasses by heat treating these glasses. The inventor has also found that a lithium electric cell and a gas sensor using the glass-ceramics exhibit excellent characteristics.
For achieving the above described objects of the invention, there are provided lithium ion conductive glass-ceramics comprising, in mol %:
where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu
and containing Li1+X(M, Al, Ga)X(Ge1xe2x88x92YTiY)2xe2x88x92X(PO4)3 (where 0 less than Xxe2x89xa60.8 and 0xe2x89xa6Yxe2x89xa61.0) as a predominant crystal phase.
In one aspect of the invention, there is provided a solid electrolyte for a lithium electric cell using these lithium ion conductive glass-ceramics.
In another aspect of the invention, there is provided a solid electrolyte for a gas sensor using these lithium ion conductive glass-ceramics.
In another aspect of the invention, there is provided a lithium electric cell comprising a case, a negative electrode, a positive electrode ad a solid electrolyte, said negative electrode, positive electrode and solid electrolyte being disposed in the case in such a manner that the negative electrode opposes the positive electrode through the solid electrolyte wherein said solid electrolyte is made of lithium ion conductive glass-ceramics comprising in mol %:
where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu
and containing Li1+X(M, Al, Ga)X(Ge1xe2x88x92YTiY)2xe2x88x92X(PO4)3 (where 0 less than Xxe2x89xa60.8 and 0xe2x89xa6Yxe2x89xa61.0) as a predominant crystal phase.
In still another aspect of the invention, there is provided a gas sensor comprising a case, a negative electrode, a positive electrode, a solid electrolyte and a layer for which an electromotive force corresponding to the concentration of the gas is produced between the two electrodes, a lead connected to the negative electrode and a lead connected to the positive electrode, said negative electrode, positive electrode and solid electrolyte being disposed in the case in such a manner that the negative electrode opposes the positive electrode through the solid electrolyte wherein said solid electrolyte is made of lithium ion conductive glass-ceramics comprising in mol %:
where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu
and containing Li1+X(M, Al, Ga)X(Ge1xe2x88x92YTiY)2xe2x88x92X(PO4)3 (where 0 less than Xxe2x89xa60.8 and 0xe2x89xa6Yxe2x89xa61.0) as a predominant crystal phase.
In the description to follow, the compositions of the glass-ceramics made according to the invention are expressed on the basis of compositions of oxides as in the base glasses. Reasons for selecting the above described content ranges of the respective ingredients and a method for manufacturing the glass-ceramics will now be described.
By melting and cooling the base glass having the above described composition and heat treating the glass to cause the crystal phase of Li1+X(M, Al, Ga)X(Ge1xe2x88x92YTiY)2xe2x88x92X(PO4)3 (where 0 less than Xxe2x89xa60.8 and 0xe2x89xa6Yxe2x89xa61.0) to precipitate, dense glass-ceramics exhibiting a high lithium ion conductivity at room temperature which was never attained in the prior art ceramics were obtained. It has been found that the same crystal phase can be precipitated also in a composition range outside of the above described composition ranges but the ratio of this crystal is so low that lithium ion conductivity of such glass-ceramic is not sufficiently high for a practical use.
Among the ingredients described above, the effect of M2O3 (where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) is particularly important. A three-part system of P2O5xe2x80x94GeO2xe2x80x94Li2O which does not contain the M203 ingredient can be glassified but melting property and thermal stability of the glass are poor and conductivity of glass-ceramic obtained by heat treating this glass is a low one of 10xe2x88x928 S/cm or below.
By adding the M2O3 ingredient, the melting property and thermal stability of the glass are significantly improved. Difference between Tx (crystallization temperature of glass) and Tg (transition temperature of glass) is generally used for estimating thermal stability of the glass and the glass becomes more stabilized thermally as this difference becomes larger. As will be shown in Examples of the present invention, a result was obtained in which Txxe2x88x92Tg increased from 54xc2x0 C. to 110xc2x0 C. by the addition of M2O3.
It has also been found, surprisingly, that conductivity of the glass-ceramics after heat treatment increases by one figure or over by the addition of M2O3. For obtaining an excellent conductivity, the amount of M2O3 should be 10% or less. If the amount of this ingredient exceeds 10%, melting property and thermal stability of the mother glass are reduced rather than increased and conductivity of the glass-ceramics after heat treatment is also reduced. A preferable range of the M2O3 ingredient is 0.1-8% and, a more preferable range thereof is 0.5-6%.
The P2O5 ingredient is an essential ingredient for forming glass and it is also an ingredient which forms the conductive crystal phase of the glass-ceramics. If the amount of this ingredient is below 30%, difficulty arises in vitrifying whereas if the amount of this ingredient exceeds 45%, the conductive crystal phase does not grow from the glass and desired characteristics cannot be achieved.
GeO2 and/or TiO2 contributes to forming of glass and is also an ingredient which forms the conductive crystal phase. In both the glass and glass-ceramics, the two ingredients can be substituted by each other at any ratio within a continuous range. For glassifying, either of these ingredients must be added and, for enabling the conductive crystal phase to grow as a predominant crystal phase from the glass and thereby achieving a high conductivity, the total amount of GeO2 and TiO2 must be within a range from 25% to 50%. A preferable range is 0-45% for GeO2 and TiO2 respectively and 25-45% for GeO2+TiO2. A more preferable range is 0-40% for GeO2 and TiO2 respectively and 28-40% for GeO2+TiO2.
The Li2O ingredient is an essential ingredient for providing Li+ ion carrier and thereby realizing the lithium ion conductivity. An excellent conductivity can be provided by addition of this ingredient in the range from 10% to 25%.
ZrO2 is effective in enhancing precipitation of the above described crystal phase. If the amount of this ingredient exceeds 8%, melting property and thermal stability of the base glass are significantly deteriorated and making of the glass thereby becomes difficult. The upper limit of this ingredient therefore is 8%. A preferable range is 6% or below and a more preferable range thereof is 5% or below.
SiO2 is effective in increasing melting property and thermal stability of the base glass and forming solid solution of Si4+ ion in the crystal phase and thereby improving the lithium ion conductivity. If, however, the amount of this ingredient exceeds 15%, conductivity is deteriorated rather than improved. The upper limit of this ingredient therefore is 15%. A preferable range of this ingredient is 13% or below and a more preferable range thereof is 10% or below.
Al2O3 and/or Ga2O3 are effective in improving melting property and thermal stability of the base glass and forming solid solution of Al3+ and/or Ga3+ion in the crystal phase and thereby improving the lithium ion conductivity. If, however, the amount of each of these ingredients exceeds 12%, thermal stability of the glass is deteriorated rather than improved and conductivity of the glass-ceramics is also reduced. The upper limit of each of these ingredients therefore is 12%. A preferable range of each of these ingredients is 11% or below and a more preferable range thereof is 10% or below.
For improving melting property of the glass, B2O3, As2O3, Sb2O3, Ta2O5, CdO, PbO, MgO, CaO, SrO, BaO and ZnO may be optionally added. Each of these ingredients should be added in an amount not exceeding 5%. If the amount exceeds 5%, conductivity decreases with the amount of addition of these ingredients.
A method for manufacturing the lithium ion conductive glass-ceramics of the present invention will now be described.
Starting materials are weighed at a predetermined ratio and mixed uniformly and the mixed materials are put in a platinum crucible and heated and melted in an electric furnace. First, gas components coming from the raw materials are evaporated at 700xc2x0 C. and then the temperature is raised to 1300xc2x0 C. to 1450xc2x0 C. and the materials are melted at this temperature for about one to two hours. Then, the melt is cast onto a stainless steel plate to form sheet glass. The resultant glass is subjected to heat treatment within a temperature range from 600xc2x0 C. to 1000xc2x0 C. for one to twenty four hours and lithium ion conductive glass-ceramics containing Li1+X(M, Al, Ga)X(Ge1xe2x88x92YTiY)2xe2x88x92X(PO4)3 (where M is an element or elements selected from the group consisting of In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) as a predominant crystal phase and exhibiting a high lithium ion conductivity is thereby provided.
The glass-ceramics of the present invention can be manufactured also by the following method.
Starting materials are weighed at a predetermined ratio and mixed uniformly and the mixed materials are put in a platinum crucible and heated and melted in an electric furnace. First, gas components coming from the raw materials are evaporated at 700xc2x0 C. and then the temperature is raised to 1300xc2x0 C. to 1450xc2x0 C. and the materials are melted at this temperature for about one to two hours. Then, the melt is cooled in water to produce glass. The resultant glass is crushed with a ball mill and passed through a sieve of 150 mesh to provide glass powder. The glass powder is press-formed and put in an electric furnace to be heated at 600xc2x0 C. to 1200xc2x0 C. for one to twenty four hours. Thus, glass-ceramics containing the above described crystal phase as a predominant crystal phase and having a high lithium ion conductivity are provided.